feat: expose SetTxDecoder

[rootulp]

Part of celestiaorg/celestia-app#1620

[evan-forbes]

for posterity, the other option to this is pulling out the tx config in the auth module, but that requires exporting some of the sign mode handlers and the wrapper struct that is used for tx building. If we do that, then we will likely need or want to pull out the entire auth module.

This seems like the simplest solution to celestiaorg/celestia-app#1665